Yesterday I installed Maui-Feiry browser, but it seems the maui app has its own theme and doesn’t take over the plasma theme. I don’t find any documentation about the fact it should take over the plasma theme yes or no.
More about MauiKit on their https://mauikit.org/ page. The Maui HIG is a different concept based on their own Framwork Maui / MauiKit · GitLab
It does integrate with KDE Plasma, as is based on Kirigami and QCC2, but Breeze is out of question. You will need the nitrux aka
nx-plasma-look-and-feel and related packages (you can find on repository too) or on GitHub - nx-desktop/nx-plasma-look-and-feel: NX Look and Feel package to make things look alike, or go via the maui-shell GitHub - Nitrux/maui-shell: Maui Shell is a convergent shell for desktops, tablets, and phones.